igneous

Obsidian

Obsidian (volcanic glass), amorphous SiO2 + impurities

Hardness: 5-5.5 (Mohs scale); Color: Black, deep brown, or grey; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Hardness: 5-5
Color: Black, deep brown, or grey
Luster: Vitreous/glassy

Identified March 15, 2026More igneous →

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Physical properties

Hardness: 5-5.5 (Mohs scale); Color: Black, deep brown, or grey; Luster: Vitreous/glassy; Structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.4

Formation & geological history

Formed as an extrusive igneous rock through the rapid cooling of silica-rich (felsic) lava, preventing crystal growth. It is relatively young geologically (mostly Cenozoic) because the glass eventually devitrifies into crystals.

Uses & applications

Used historically for sharp tools and weapons (scalpels, arrowheads); modern use in jewelry as a gemstone, decorative carvings, and in high-end surgical blades.

Geological facts

Because it breaks with a conchoidal fracture producing edges thinner than a steel scalpel, obsidian blades are still occasionally used in specialized cardiac and eye surgeries to minimize scarring.

Field identification & locations

Identify in the field by its smooth, glassy texture and distinctive curved, shell-like (conchoidal) fractures. Found in volcanic regions like the American Northwest, Mexico, Iceland, and Italy. Collectors look for 'sheen' varieties like gold or rainbow obsidian.

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Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

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Magnetite

Magnetite (Fe3O4)

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